24lb injectors and fuel pressure

Founding Member

I just swapped some 24lb injectors and a Pro-M mass air meter onto my car. Is there any kind of adjusting that you have to do to the fuel pressure when you install these? It seems like you would have to turn up the fuel pressure a little bit to accomodate the larger injectors.

The reason I ask is after the install, my car runs pretty lousy once it is all warmed up, a lot worse than before the injectors. (lean/rich problem?)

Founding Member

If you don't have aftermarket heads, intake, and cam, it's likely you'll be running wayyyyy too rich. Get an adjustable fuel pressure regulator and adjust it to around 32-34 psi. That should be about right, considering you're running too big of an injector for your current setup...

Founding Member

You're supposed to set the pressure with the vacuum line disconnected from the regulator. If you're at idle, then there is alot of vacuum in the intake and that in turn lowers the fuel pressure automatically. The pressure you set the regulator to only happens at WOT when there is little vacuum in the manifold.

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You set the pressure with the vac line disconnected from the regulator, 34-38lb is stock.

If your car is lean up top at WOT, then you can add fuel with the AFPR go in 1/2 lb increments.

If your car is rich down low it is not the injector, you can run a 42# on a stock car, yes the spray pattern is terrible and it will not run very good but the EEC and O2's give the feedback and the EEC will adjust the pulse with adn cycle time of the injector to try to make it right which is called "stoich"

DREW Set it up to 36lbs VAC OFF and see if it runs any better...usually they drop around 9lbs with the vac attached.

Founding Member

Funny, my setup with 30#ers, AFR 165 heads, Crane 2031 cam, TFS Street, and all the other goodies wouldn't idle for jack with the 30s unless the fuel pressure was set to 28-32 psi with the vacuum off. It would surge, load up on fuel, and then die if I slowly turned up the pressure with the vacuum off at idle. At 28 psi., I didn't get any pressure drop, like I should have, when I reconnected the vacuum line. The AFPR was maxed out on the adjustment. I think he'll have to really lean fuel pressure down to see a good idle. The fuel pressure at idle DOES make a difference. The ECU is relying on pre-programmed curves to determine what the optimal injector pulse width is, based on all of its feedback. I think a big injector can lead to a situation where the ECU cannot physically compensate enough... JMHO...

When I went to 24s, which is a more correctly sized injector, the car idled perfect, even before I reprogrammed the transfer function in my MAF Tuner for the new injector size...

haha, i guess my car is tolerable, i didnt have a gauge and when i got the car i was running atleast 65 psi on fuel pressure... didnt act funny, started and idled fine but allitle lopey, i think that is from the cam though. Kindof funny mine didnt screw up with that much fuel pressure. im at around 38-40 right now and i think that is fine with my setup